The slot relieving of wellside zone of producing beds is known as one of the most effective methods for increasing productivity (output) of oil, gas, fill-in, hydrogeological, engineer-geological or water-supplying well.
This method provides creating of slots in the wellside zone, wherein the width, depth and orientation of the slots are predefined by known methods according to characteristics of the well and productive bed.
Practical cutting of slots having predetermined parameters is a very difficult technical problem, since the cutting is performed in complicated conditions of various rocks, temperatures, at great depth, in the presence of extracted product and/or washing liquids in the welbore, with remote control and monitoring only. One of the most successful techniques for cutting slots is the sandblasting hydro-abrasive perforation, in which the cutting is performed with a jet of water and sand at a very high pressure of liquid. Apparatus for implementing this technique is shown in close-up in FIG. 1. It comprises a tubing (pipe) 1 which is rigidly joined with a housing tube (housing pipe) 2, within which an inner tube 4 is moveably mounted coaxially relative to housing 2 (and to a gap 3), the inner tube being spring-loaded from below with a cock spring 5. A ball valve 6 is mounted at the edge of inner tube 4, and a perforator 7 is mounted above that valve. In the inter tube gap 3, a hydro-brake 8 is formed comprising two vertical hermetic chambers 9 and 10 constrained by plugs 11 and 12. The chambers are filled with liquid and separated by a piston 14 having a channel 15 for cross-flow of liquid between the chambers, the top plug 11 being rigidly joined with the case 2 and moveably joined with the inner tube 4. The piston 14 is rigidly joined with the inner tube 2 and moveably joined with the case 2. The bottom plug 12 is moveable relative both tubes.
The apparatus in FIG. 1 operates as follows. The outer tube 2 is jointed by means of pipe coupling (like a collar) with the tubing 1. The second end of tubing 1 is connected to a pump unit of the equipment on the surface. The top chamber 9 is filled with viscous liquid, wherein the amount of liquid for each cutting process is chosen in accordance with the slot length and depth calculated in advance, based on the known parameters of the channel 15 (capillary) connecting the chambers 9 and 10, temperature dependence of the sizes of the channel and the fluidity of liquid, as well as the pressure of the pump unit.
Then the housing 2 of the apparatus (along with the inner tube 4, hydro-brake 8, cock device 5, hydro-abrasive perforator 7, and ball valve 6 disposed therein) is lowered to the predetermined depth, so that nozzles of hydro-abrasive perforator 7 are in the place of the well where the top edge of the formed slot should be cut. A ball is dropped into the inner tube 4 (in order to close the ball valve 6). The pump unit of the surface equipment is then turned on, and it begins to pump the abrasive mixture with the predetermined (rated) pressure into the inner tube 4. The mixture passes down to the ball valve 6 and closes it tightly. The hydro-abrasive perforator 7, via nozzles, begins to cut slots, first in the casing walls, then in the wellside zone. The depth of cutting of a future slot depends, in particular, on the time during which the cutting is performed. Pressure within the inner tube 4 (through which the abrasive liquid is pumped under pressure) pushes the inner tube 4 down and out of the outer tube (housing) 2 rigidly mounted on the tubing 1. A slow cross-flow of liquid from the top chamber 9 of hydro-brake 8 into the bottom chamber 10 occurs, predetermined by the time of the cross-flow defined by the channel 15. Thus, the inner tube 4 with the perforator 7 mounted thereon is slowly lowered down, and the rate of its movement defines the depth of (slot) cutting. At that time, only the cock spring 5 pushes from below against the plug 12 of the bottom chamber 10, which pressure is chosen essentially less than the pressure against the top plug 11. In other words, the pressure of the cock spring 5 does not impede the liquid to cross-flow from the top chamber 9 into the bottom chamber 10. This process of slow lowering (in the presence of pressure in the tubing 1) lasts until all the oil from the bottom chamber is forced to flow into the top chamber. I.e., the characteristics of liquid poured initially into the bottom chamber 9 define the height of cutting, or the future slot. The duration of the cross-flow defines the rate of lowering of the apparatus for cutting slots, i.e., the depth of cutting of such slot. If the necessary cutting height is achieved before all the oil in the top chamber cross flows into the lower chamber, then at that moment the pressure in the tubing 1 is released, and the process of cutting the slot and the liquid cross-flow is stopped. In doing so, since the pressure against the top plug 11 of hydro-brake 8 is stopped, the cock spring 5 pushes liquid out of the top chamber 9 into the bottom chamber 10 while raising the inner tube 4 into its initial position. The same process occurs after all the oil is squeezed into the bottom chamber 10, if the pressure in the tubing 1 is withdrawn at that time.
An effective example of such an apparatus for cutting slots by hydro-abrasive perforator, which apparatus being used for developing production columns with a slot and creating vertical relieving slot-shaped cavities in the wellside zone, could be the apparatus 4, the design of <<>>[Special Designing Bureau “Sevmorgeologiya”, RU], which operates in the above described manner. Known are another similar apparatuses. The closest analog is the apparatus according the U.S. Pat. No. 6,652,741 issued Nov. 25, 2003.
Now the processes taking place in the process of cutting slots by hydro-abrasion will be analyzed. Essentially in all known apparatuses of this type, a movement of the hydro-abrasive perforator 7 occurs from top to bottom by a pressure in the tube space. The pressure in the tube space is created for operation of a hydro-abrasive jet. Because of the pressure differential between the tube space and the annular space (outside the tube) at hard-alloy nozzles of the hydro-abrasive perforator 7, an abrasive particle is “charged” with energy (the particle outlet velocity from the hard-alloy nozzle reaches several meters per second). When hitting the production column wall and then the rock, a particle “performs” work: it destroys metal and rock. Hydro-abrasive perforator 7 is lowered into a well on tube 1. The main part in this apparatus is the hydro-brake 8, sealed relative to the housing 2, which is rigidly joined with the column of tube 1. The inner tube 4 sealed relative to the housing 2 can move a fixed distance limited by top plug 11 and bottom plug 12. The piston 14 is disposed in the middle of a stroke between the plugs, which piston is implemented as a seal having a capillary through hole, i.e., the channel 15. The space between the top plug 11 and the piston 14 with the capillary, which is previously referred to as the top chamber 9 of hydro-brake 8, is filled with viscous liquid. The space between the bottom plug 12 and the piston 14 with the capillary, which is previously referred to as the bottom chamber 10 of hydro-brake 8, is filled with viscous liquid. Pressure in the tube space causes the displacement of the inner tube 4 relative to the housing 2 due to the cross-flow of viscous liquid from the bottom chamber 10 into the top chamber 9. The rate of that displacement is adjusted by the length and the cross-section of the capillary, and the selection of the viscosity of the liquid and the cross-section and length of the capillary, so that the displacement rate is predetermined for the optimal operation of hydro-abrasive perforator 7 under the conditions of the face of well (the temperature and working pressure in the tube space).
The bottom seal-plug 12 is made moveable relative to the inner tube 4 and housing 2, and the bottom plug 12 is spring-loaded relative to the housing by the cock spring 5. Thus, pressure is created in the bottom chamber 10 of hydro-brake 8 of hydro-abrasive perforator 7, which pressure ensures the initial position of the bottom plug 12, and, therefore, the inner tube 4 with the hydro-abrasive perforator 7, in the upper top position. Pressure of the cock spring 5 in the bottom chamber 10 of the engine of hydro-abrasive perforator 7 (cock force) is chosen such that this force ensures the raising of hydro-abrasive perforator 7 into the upper top position not exceeding 30% of the forward stroke force exerted by the working pressure in the tube space.
More commonly, the back stroke or the cock of the engine of hydro-abrasive perforator 7 occurs due to the straightening the spring 5 which encompasses the inner tube 4 inserted between the housing of hydro-brake 8 and hydro-abrasive perforator 7. The lower end of the cock spring 5 is pushed against the perforator housing, and spring's upper end is pushed against the bottom plug 12 of the hydro-brake 8. A displacement of the inner tube 4 relative to the case 2 compresses the spring 5 because of working pressure in the tube space. Once the pressure in the tube space and the annular space is equalized, the spring 5 straightens and pushes out the inner tube 4 with the perforator 7 into the upper top position.
The signaling device 35 ensures control of the rate of displacement of hydro-abrasive perforator and provides information about completion of the working stroke of the hydro-abrasive perforator. This device is, e.g., all upward pin mounted under the ball valve 6 at a holder rigidly jointed with the housing 2. Said pin opens up the ball valve 6 at the end of the working stroke of hydro-abrasive perforator 7, at which time the pressure in the tube space drops sharply, informing of the completion of the working stroke.
Using such apparatus, it is possible to open up a production column and create slot-shaped key seats in the wellside zone of the productive bed. During operation of such apparatus in a well, a foreman gets information on the hydro-abrasive perforator displacement rate, the completion of hydro-abrasive perforator engine working stroke, the condition of the hard-alloy nozzles during the operation. There is a possibility to initiate forward or backward washing of the well at any stage of the operation in the well, supply the fraction-treating solution, drive the pressurized solution into a formation, and, when necessary, perform the hydraulic formation fracturing.
In spite of the merits of said apparatus, there are some the common drawbacks:
Unpredicted stoppages and alterations of rate (depth) of slot cutting are unavoidable, since all the known hydro-abrasive perforator engines work on the principle of a hydro-brake: the cross-flow of viscous liquids from one chamber to another through a capillary, i.e., a movement rate is set by the capillary's length and cross-section. The reason for stoppages and alterations is the attenuation of the movement of the liquid movement through the capillary. In the case of such a stoppage or essential alteration of the rate, it is necessary to raise the face equipment and readjust it. This is a very costly and lengthy operation.
Dependence of the slot forming rate on temperature. It is known that the viscosity of a liquid depends on its temperature. In order to set the rate of the movement (displacement) of hydro-abrasive perforator of the apparatus in a well during operation (i.e., under the real conditions), it is necessary to perform a series of complicated operations on the surface. The operations related to simulating the working conditions in a well and to calculating a face temperature changes during stoppages while washing the well, and at various rates of pumping in cycle and during the discharge. In addition, the accuracy of such calculations is rather low, which reflects on operation quality and reliability.
Duration of operation for opening a ball valve at the perforator edge, i.e., the operation of extracting said ball. The ball valve is opened by a backward washing process at a speed permitting to raise the ball up to wellhead and to extract it from the tube space. Depending on the depth of the operation, the operation of extracting the metal ball from the tube space requires from one to three, and sometimes five hours. It is necessary to close the ball valve in order to resume a hydro-abrasive perforator working stroke and to maintain pressure differential at the hard-alloy nozzles in the perforator. This procedure is implemented by repeatedly inserting the metal ball into the tube space. Once reaching the hydro-abrasive perforator, the metal ball blocks the aperture in its edge. Supply of working liquid—the pulp—will occur thereafter only through the hard-alloy nozzles. In order to switch to other operation, such as the forward or backward washing of well, insertion of technological solution, replacement of the pulp by a technological solution, hydraulic fracturing of formation, etc., it is again necessary to have a forward open edge of the hydro-abrasive perforator, or else all the operations having to do with the supply of the liquid into tubing 1 encounters the hydraulic resistance of the hard-alloy nozzles.
Presence of signaling device in the lower part of the face equipment, in fact, shuts off the “forward” flow of the liquid from the hydro-abrasive perforator's edge. It is impossible to perform cleaning of a sump in the well from the accumulated sludge and sand, and it is impossible to place an instrument onto the face in order to packer release it and for other purposes.
In other words, the known apparatuses are insufficiently efficient, insufficiently convenient, reliable, stable in time, and they are temperature dependent.